Conventional systems for selectively heating or cooling a particular medium, such as air, require energy from limited sources and which consequently is increasingly more expensive. Much attention has been given, particularly in recent times, to sources of energy which exist as natural phenomena. Such energy includes solar energy, geothermal energy, tidal energy and wind-generated energy. While all of these energy sources have advantages and disadvantages, the geothermal energy appears to be most reliable, readily available and most easily tapped.
It is common knowledge that the center of the earth is extremely hot and that a skin of very dense rock separates the hot interior from the surface of the earth. Although this rock in the earth's skin is resistant to passage of heat, nevertheless a small amount of heat per unit area is transmitted from the hot interior of the earth to the surface of the earth. The amount of heat transmitted from the center of the earth to the surface is found to be ##EQU1## or 15.9 BTUs per square foot per hour.
The heat so transmitted through the earth's skin is absorbed by subterranean water which flows through a system of veins. These veins practically blanket the surface of the earth adjacent to the rock skin and vary in depth from the surface of the earth from approximately 100 feet to several hundred feet. In the Piedmont section of North Carolina and South Carolina, the depth is usually from 100 to 200 feet and the temperature of the water is substantially constant at approximately 60 degrees F. This temperature of the subterranean water is maintained by loss of heat by conduction to the surface of the earth. The earth, stone, etc. between the subterranean water and the surface of the earth are fairly good conductors of heat because of the presence of moisture in the soil. Therefore, the input of heat from the center of the earth is balanced by the loss of heat, over a considerable time, from the subterranean water through the surface of the earth.
Many proposals have been made to utilize this heat as a source of geothermal energy with varying degrees of success. Where, through faults in the rock layer separating the very hot core from the earth's surface, heat sources of very high temperatures are near the earth's surface, considerable success has been experienced in utilizing such heat in electric power generation and for other uses. However, where no such faults exist, considerably less success has been obtained--although many different approaches have been suggested.
Among these prior suggestions have been systems which employ coils, tanks or other liquid containers buried in the earth at depths ranging from open containers extending downwardly from the surface to a depth of several feet to coils or closed tanks buried several feet beneath the surface of the earth. These systems have suffered from many deficiencies and have not been effective in tapping this virtually inexhaustible heat source. For example, these systems rely exclusively on the earth immediately surrounding the liquid containers and the heat that can migrate thereinto. This earth or surrounding soil has a decided tendency to become dry as heat is transferred between the soil and container and when dry becomes a thermal insulator around the container. Additionally, such coils, tanks, containers and the like are usually constructed of plastic or other materials which are not efficient thermal conductors and have other inefficiencies inherent or incorporated therein.
Another prior proposal has suggested the use of subterranean water from relatively deep wells as the source of geothermal heat transfer. The main problems with this approach are the disposal of the well water after the heat transfer has occurred, and the energy required to transfer water from the well to the heat pump.